The present invention relates to a method of upgrading waste material comprising sheet metal with a zinc plating on at least one side thereof.
This sheet metal shall be referred to as zinc plated sheet metal in the present text.
The application of the present invention is most particularly found in the recovery and the upgrading of zinc plated sheet metal waste material produced by the automobile industry during the manufacture of the various pieces of cars consisting of said sheet metal, in particular during the operations of pressing and cutting out of said sheet metal.
Currently, the automobile industry consumes, for the manufacture of the bodywork and the doors of cars in particular, increasing amounts of pressing sheet metal plated with a zinc layer for protecting said sheet metal against corrosion.
The various operations during which the bodywork is manufactured from sheet metal necessarily lead to the production of relatively significant amounts of waste material known as "fresh scraps" from the automobile industry.
This waste material consists of fragments of zinc plated sheet metal of various sizes and having already optionally undergone a certain number of operations of pressing and cutting-out. These are therefore fragments of zinc plated sheet metal but which are in general far from being flat.
Furthermore, this waste material, given the medium and the conditions in which they are produced, are in general more or less soiled by traces of grease.
The method of the present invention is addressed to this type of waste material which is initially piled up more or less in bulk in workshops.
The presence of a relatively significant amount of zinc on the surface of the waste material prevents the direct recycling of this waste material in foundries or in steel works.
The re-use of the zinc plated sheet metal waste material does in fact pose various problems whatever the type of steel works in which recycling of these waste materials is tempted.
In a general manner, the zinc plated sheet metal used in the automobile industry in fact contains on average from 12 to 24 kilograms of zinc per tonne of sheet metal.
Melting down of the waste material cannot take place without causing various problems linked on the one hand to the respect of good conditions of hygiene and of security, and on the other hand to the resistance of the materials used to carry out this melting down.
More specifically, the presence of zinc makes it necessary:
to aspirate and treat the fumes, PA1 to control the concentration of zinc in the dust collected in the filters, PA1 to control the functioning of the filters under the conditions to which they are submitted, PA1 to capture the films, and this has certain difficulties, PA1 to prevent any problem of pulmonary and digestive irritation in persons working in melting workshops. PA1 electric steel works, PA1 conversion steel works, PA1 induction furnace foundries. PA1 a) a first step referred to as mechanical processing step of said waste material to improve the accessibility of said zinc layers to a subsequent chemical etching, by increasing the accessible etching surface of the parts forming the zinc layers, on the one hand by carrying out the separation of said sheet metal from each other by the crushing of said waste material and, on the other, by creating cracks in said zinc layers, PA1 b) a second step referred to as chemical etching to dissolve the zinc, by the immersion of said waste material in a basic bath, PA1 c) a third step of separation of the zinc-free sheet metal from the chemical etching bath having dissolved the zinc.
Furthermore, it is well known that the formation of a eutectic ZnO/SiO.sub.2, the melting point of which is 1342.degree. C. may be the origin of a degradation of the fireproofing materials constituting the lining of the furnaces in foundries.
Furthermore, it is also known that the presence of liquid zinc risks to harm the silica powder-based coatings very badly that are often found in centrifugation shells.
More specifically, three sorts of steel works are distinguished:
The melting down of the zinc plated sheet metal waste material in electric steel works give rise to relatively few problems as regards the installations themselves.
The principal problem however finds itself at the level of the dust wherein the zinc is found concentrated, this necessitates a very costly treatment.
The melting down of zinc plated sheet metal scrap in induction furnace foundries itself also gives rise to problems related both to the fireproofing material and to the emission of fumes in the sites.
Hence, the automobile industry finds itself confronted with a grave problem of recycling of the waste material constituted of zinc plated sheet metal and this, whatever type of steel works or foundry for which the recycling of the waste material is envisaged.